When you're single, flirtation can be a harmless pastime. As long as you and your flirt friend are on the same page, then it's fun to engage in a little banter now and then. Once you're in a committed relationship, however, flirting can feel more like a threat. But you can usually spot these flirtationships before they become a real problem by knowing the signs your spouse is flirting with other people.
Now it must be said that the definition of appropriate and inappropriate flirting will vary a bit for every relationship. Some people may be flattered when outsiders express an interest in their partner, while others will feel threatened when their partner's gaze lingers on someone else for too long. It's all about knowing your own comfort zone, communicating that to your parter, and trusting that your SO will respect your boundaries. As long as everything is above-board and no one gets unreasonably possessive or jealous, then the boundaries for flirting can be easy enough to establish.
That said, relationships ebb and flow over time, and the boundaries you and your SO once adhered to firmly may start to become less defined. But by recognizing the signs your partner is getting flirty with others, you can address any concerns before they turn into full-blown problems.